Group A Streptococcus (S. pyogenes or GAS) is a major public health threat in the United States and globally. It is the cause of streptococcal pharyngitis (strep throat), infections of the skin and soft tissues, and less common but potentiall fatal syndromes such as necrotizing fasciitis and streptococcal toxic shock as well as the post-infectious complications of glomerulonephritis and acute rheumatic fever, which together account for an estimated 500,000 deaths annually. The absence of a vaccine and ongoing morbidity and mortality despite treatment highlight the need for alternative approaches. An attractive novel target is a bacterial regulatory system that controls virulence. The CsrRS (or CovRS) two-component system regulates expression of 10 to 15% of the GAS genome including multiple virulence factors. During the previous funding period, we identified the human antimicrobial peptide LL-37 as a potent signal that acts through CsrRS to upregulate virulence factor expression. We propose that CsrRS acts as a sensor of host innate immunity by detecting subinhibitory concentrations of LL-37 and, in response, induces expression of antiphagocytic factors that promote invasive GAS infection. To enable the design of strategies that exploit CsrRS signaling to block virulence gene expression, fundamental questions must be answered: How is LL-37 signaling transduced to regulate target gene expression? How does the CsrR function as an activator of certain genes and a repressor of others? Why are CsrRS signal transduction mutations over-represented in invasive clinical isolates? The Aims of the research are (1) to characterize the effects of LL-37 signaling, CsrS-mediated signal transduction, and CsrR transcriptional regulation on GAS global gene expression; (2) to determine the molecular basis of CsrRS-mediated activation or repression of target gene expression; and (3) to investigate the selection bias for CsrRS-signaling mutants in invasive GAS infections. Achieving these Aims will shed new light on the molecular basis of signaling and gene regulation by CsrRS and other bacterial regulatory systems and will enable novel future strategies for prevention and treatment of infections due to GAS and other bacterial pathogens.